Inorganic chromates are frequently employed as corrosion inhibiting materials in industrial waters, particularly in circulating water systems for cooling towers. The chromates are sometimes used alone but frequently are employed in conjunction with molecularly dehydrated phosphates, the combination functioning as scale and corrosion inhibitors to protect metal conduits and other metallic surfaces with which the circulating water comes in contact.
Water being used as a cooling medium in industrial applications frequently is very high in dissolved solids, usually exceeding 1000 parts per million or so. When a cooling tower which has been treated with chromate corrosion inhibitors reaches a certain concentration level of dissolved solids, it is customary to blow it down by discharging a portion of the water and adding fresh makeup water having a lower dissolved solids content. A typical cooling water blow-down might contain, for example, 20 parts per million of a chromate and 1200 parts per million of other dissolved salts such as sodium chloride, calcium sulfate, magnesium nitrate, and the like.
When such waters are discharged into natural water courses or to sewage systems, the high chromate concentration may pose a problem because of local regulations covering the nature of effluents which can be discharged. The presence of the large dissolved solids content, however, makes it quite difficult to remove the chromate ions selectively while leaving the relatively innocuous salts for discharge.